Degree Type

Dissertation

Date of Award

2006

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

L. Scott Chumbley

Abstract

Magnetic refrigeration is a promising technology that offers a potential for high energy efficiency. The giant magnetocaloric effect of the R5(Six,Ge1-x)4 alloys (where R=rare-earth and 0≤x≤1), which was discovered in 1997, make them perfect candidates for magnetic refrigeration applications. In this study the microstructures of Gd5(SixGe1-x)4 alloys have been characterized using electron microscopy techniques, with the focus being on distinctive linear features first examined in 1999. These linear features have been observed in R5(Six,Ge1-x) 4 alloys prepared from different rare-earths (Gd, Tb, Dy and Er) with different crystal structures (Gd5Si4-type orthorhombic, monoclinic and Gd5Ge4-type orthorhombic). Systematic scanning electron microscope studies revealed that these linear features are actually thin-plates, which grow along specific directions in the matrix material. The crystal structure of the thin-plates has been determined as hexagonal with lattice parameters a=b=8.53A and c=6.40A, using selected area diffraction (SAD). Energy dispersive spectroscopy analysis, carried out in both scanning and transmission electron microscopes, showed that the features have a composition approximating to R5(Six,Ge1-x )3.phase. Orientation relationship between the matrix and the thin-plates has been calculated as [-1010](1-211)p//[010](10-2) m. The growth direction of the thin plates are calculated as (22 0 19) and (-22 0 19) by applying the Deltag approach of Zhang and Purdy to the SAD patterns of this system. High Resolution TEM images of the Gd5Ge4 were used to study the crystallographic relationship. A terrace-ledge structure was observed at the interface and a 7° rotation of the reciprocal lattices with respect to each other, consistent with the determined orientation relationship, was noted. Both observations are consistent with the stated hypothesis that the growth direction of the thin-plates is parallel to an invariant line direction. Based on the terrace-ledge structure of the thin-plate interface a displacive-diffusional growth mechanism has been proposed to explain the rapid formation of the R 5(Six,Ge1-x)3 plates.

DOI

https://doi.org/10.31274/rtd-180813-14285

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu

Copyright Owner

Ozan Ugurlu

Language

en

Proquest ID

AAI3217324

File Format

application/pdf

File Size

102 pages

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